Abstract
In western North America, an increase in drought frequency and wildfires has raised concerns about the ability of forest ecosystems to recover. It is critically important to understand how multiple disturbances can affect the physiology and mortality of trees, especially smaller trees (saplings), which are not represented in existing fire-effects predictive models. We investigated the impact of fire intensity on Pinus monticola, Pseudotsuga menziesii, Picea engelmannii, and Thuja plicata saplings by evaluating physiological, morphological, and mortality responses. Using a pyro-ecophysiology experimental approach, saplings were subjected to a range of controlled surface fire intensities, and assessments of physiology, morphology, and mortality were observed up to 42 weeks post-fire.
In these studies, we demonstrated that drought increases mortality in saplings across all fire intensities, and post-fire chlorophyll fluorescence (Fv/Fm) serves as an effective early warning sign of death. We developed species-specific optimal prediction models: for P. monticola, post-fire crown volume scroched, while for P. menzeisii, the model used flame height and FRE, which showed delayed mortality up to 42 weeks post-fire. Then, P. engelmannii and T. plicata, which are considered fire-vulnerable as mature trees, whereas saplings were found to be as fire resistant as P. menziesii at higher intensities. A new general sapling mortality model (R2 = 0.89) was derived from seven conifer species to improve current crown scorch-based models that overestimate mortality at low to moderate fire intensities.
This dissertation demonstrated that pre-fire physiological conditions drive post-fire recovery; for early warning of mortality, use chlorophyll fluorescence trajectories; and that current predictive fire models need updating to include small-diameter trees. Natural resource managers can use these findings to improve predictive tools for prescribed fire planning and post-fire replanting decisions in response to changing fire regimes